The objectives of this research are to study the induction of mutations by low doses of radiation in cultured mammalian cells and to determine how the dose-response relationship is affected by dose rate, the type and chromosome location of the target mutation, and the repair capacity of the host cell. Strains of L5178Y mouse lymphoma cells (LY-R and LY-S) which differ from each other in their sensitivity to the lethal and mutagenic effects of radiation will be used for this study. The frequency of radiation-induced mutation will be measured at the thymidine kinase (TK) locus using sublines of LY-R and LY-R cells which are heterozygous at the TK locus and which are highly mutable by radiation. We will determine if mutation frequency increases linearly with dose in the low-dose range and whether the frequency is affected by changes in the dose rate and/or the repair capacity of the cells. The extent to which multilocus lesions are produced in the two strains following exposure to high dose-rate and low dose-rate radiation will be determined on the cellular, chromosomal, and molecular levels, by (a) measuring the colony size of TK -/- mutants growing in selective medium; (b) carrying out cytogenetic analyses of the radiation-induced TK -/- mutants; and (c) analyzing the copy number, the structure, and the activity of the TK gene, using labeled TK cDNA clones. We have found that the radiation-induced mutability of the TK gene is higher than that of the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) gene and that the mutability of these genes varies markedly among the different cell strains. The relationship of these differences to the occurrence of radiation-induced multilocus lesions will be investigated by comparative analysis of the gene structure of the radiation-induced mutants, using labeled cDNA clones, as outlined above. Molecular mechansims responsible for the sensitivity of strain LY-S to ionizing radiation will be studied by searching for repair deficiencies in this strain as compared to strain LY-R. The results of these experiments should be of value in the estimation of radiogenic risks from exposure to low doses and/or low dose rates of ionizing radiaiton. Further, the results should reveal factors which affect the dose-response relationships and yield information as to the molecular mechanisms involved in radiation-induced damage leading to lethality, mutation, and oncogenesis.